A principal hazard of use of pneumatic tires has always been punctures by sharp objects such as nails or splinters of glass. Of the many proposals for sealing punctures, the most successful have been sticky materials on the inside of tires, but they have been difficult to handle in the manufacture of tires and their expense has appeared to be greater than the benefits would justify. Layers of sponge or cellular rubber have also been proposed but seem never to have gone into practical use, perhaps because the proponents did not understand the conditions required for sealing with such materials and consequently could not obtain satisfactory results.